Electric microgauge system



C. F. COAKE ELECTRIC IIICBOGAUGE SYSTEM Nov. 8, 1949 2 sneaks-sheet 1Filed Feb. 16, 1945 INVENTOR CHAfiLES A" (OAK:

ORNEY Nov. 8, 1949 c. F. COAKE ELECTRIC MICROGAUGE SYSTEM 2 Sheets-Sheet2 Filed Feb. 16, 1945 J Y INVENTOR (H/KY5; K-"(/M'L BY I ORNEY PatentedNov. 8, 1949 ELECTRIC DIICROGAUGE SYSTEM Charles F. Coalre,Indianapolis, Ind., nsignor to Senn Corporation, New Augusta, Ind., acorporation of Indiana Application February 16, 1945, Serial No. 578,320

1 Claims. 1

This invention relates to electric microgauges of the type including avariable impedance device the impedance of which is varied in accordancewith the quantity to be measured, and more particularly to a novel andimproved system for actuating an instrument in response to such changesin impedance.

An object of the invention is to provide a system of the above typehaving improved characteristics of sensitivity and dependability.

Another object is to provide an instrument of the above type which maybe readily adjusted for diflerent ranges of indication.

Another object is to provide a system of the above type having aplurality of measuring heads and circuit means for switching from onehead to the other without any readjustment of the circuit constants.

Another object of the invention is to provide a system for making directmeasurements of unknown values of resistance, capacity and inductance.

Various other objects and advantages will be apparent as the nature ofthe invention is more fully disclosed.

In accordance with the present invention the variable impedance deviceis connected in circuit with a pair of space discharge devices operatingas a push-pull oscillator and locked in step by suitable coupling means.The variable impedance devices are so connected in the circuit that whenthey are of predetermined impedance the grids of the two oscillatortubes are operated at the same bias and the tubes draw equal spacecurrents. Any change in the relative impedances of the two devices,however, produces unequal grid voltages and causes the tubes to drawunequal space currents. A measuring device such as a center indicatingmicro-voltmeter is connected to measure the unbalance between the spacecurrents drawn by the two tubes.

It has been found that a circuit of the above type is extremelysensitive to minute changes in impedance of the variable impedancedevice and that indications of extreme sensitivity may be obtained.

In order to adapt the circuit for measurement of variables havingdifferent ranges, a pair of output resistance networks are providedwhich are connected between the outputs of the oscillator tubes and theindicating instrument. Switching means is arranged to connect thedifferent networks into the circuit in accordance with the range ofvariables to be measured.

In order to adapt the system for use with a plurality of measuring headsas for example in a template for measuring different parts of an object,a separate set of output and adjusting networks is provided for eachhead and switching means is arranged to connect into the circuit thenetwork which corresponds to the particular head being used formeasurement. In this way each network may be properly adjusted inaccordance with the constants of the respective heads so that themeasuring instrument may be instantly switched from one measuring headto the other without stopping to make intermediate adjustments of thecircuit.

Another feature of the invention resides in the provision of means formaking a direct measurement of unknown resistances, capacities andinductances. These elements are switched into corresponding parts of thecircuit to replace one of the matched elements of the push-pulloscillator and are measured by making suitable ad- Justments of thecorresponding matched elements to produce balanced operating conditionsin the oscillator as indicated by a zero indication on the indicatinginstrument.

Although the novel features which are believed to be characteristic ofthis invention are pointed out more fully in the claims, the inventionwill be better understood by referring to the following descriptiontaken in connection with the accompanying drawings in which specificembodiments thereof have been set forth for the purposes ofillustration.

In the drawings,

Fig. 1 is a schematic diagram of a simple circuit illustrating the basicprinciples of the present invention;

Fig 2 is a schematic diagram illustrating an adaptation of the circuitof Fig. 1 to the measurement of unknown values of resistance, capacityand inductance; and

Fig. 3 is a schematic diagram of a measuring circuit incorporating aplurality of measuring heads and a plurality of resistance networkscorresponding to the different heads and to the difierent ranges ofmeasurements.

Although specific terms have been used herein for convenience inreferring to the various details of the invention, it is to beunderstood that these terms are to be given an interpretationcommensurate with the state of the art.

Referring to Fig. 1, the variable impedance elements are shown ascomprising a pair of coils III, II associated with a magnetic circuitincluding fixed cores l2 and I3 and an armature ll mounted for movementin response to variations in the quantity being measured and adapted tovary the reluctance of the magnetic circuits in an inverse manner foreffecting corresponding changes in the impedances of the coils Ill and IThe construction of a suitable unit including the coils I and II isshown more in detail in my copending application, Serial No. 548,281filed August 5, 1944, now Patent No. 2,466,382, for Micro-limitindicating system and only so much thereof is shown herein as isnecessary for an understanding of the present invention.

Condensers l5 and I6 are connected respectively across coils l0 and Hand are so related to the coils I0 and H as to form therewith tunedcircuits which are resonant at a predetermined frequency such as 180cycles when the variable armature I4 is in its mid-position.

The coils I0 and II are connected respectively to the grid 2|! andcathode 2|, and grid 22 and cathode 23 of space discharge tubes 24 and25 having plates 26 and 21 respectively. Blocking condensers 28 and 29and grid leak resistors 30 and 3| are connected in the respective nputcircuits. The tubes 24 and 25 may comprise a double triode or maycomprise separate triodes or multi grid tubes as desired.

The plates and grids of the tubes 24 and 25 are coupled by crossconnections including condenser 32 and resistance 33 connected in seriesbetween the grid 20 of the tube 24 and the plate 21 of the tube 25, anda condenser 34 and resistance 35 connected in series between the grid 22of the tube 25 and the plate 26 of the tube 24.

The plates 26 and 21 are interconnected by a resistance networkincluding fixed resistors 38 and 39 and a potentiometer 40 is connectedin series. The tap 4| of the potentiometer 40 is connected through asource of plate potential, shown as a battery 42, to the cathodes 2| and23. It is to be understood that the tubes 24 and 25 may be provided withsuitable heaters, control elements, and sources of biasing potential notshown.

A potentiometer 45 is connected in series with a choke 46 across theresistance network includ ing the resistors 38, 39, and 40. A measuringinstrument 41, such as a center reading millivoltmeter, is connectedacross one side and a variable tap 48 of the potentiometer 45. a

In the above circuits the resistances 33 and 35 and the condensers 32and 34 constitute feedback networks which cross connect the oscillatortubes to cause' them to operate in unison and in a push-pullrelationship, the plate of one tube being 180 out of phase with respectto the grid of the other tube. The tuned grid circuits cause the tubesto oscillate at the predetermined frequency. In addition to thefeed-back network the tubes are coupled through the mutual inductance ofthe coils l0 and II due to the armature l4 and the assoc ated magneticcircuits and are thus further stabilized to operate in unison at saidpredetermined frequency.

When the coils IO and II are of equal impedance the circuits includingthe coils in and ii and the condensers l5 and I6 respectively aresimilarly tuned and the grids 20 and 22 have an equal swing and causeboth tubes to draw the same space current. Under these conditions, bothends of the resistance network 38, 39 and 40 at at the same potentialand a zero reading is obtained on the instrument 41. The potentiometer4|! is adjusted to compensate for variations in tube characteristics andto produce a zero indication on the instrument 4'! under the conditionsabove specified.

When the impedances of the coils l0 and ii are varied in an oppositesense by movement of the armature H, the tuned circuits arecorrespondingly varied and the grid potentials are changed so that thetubes 24 and 25 no longer draw the same space current, although thefrequency of oscillation is not appreciably changed. The potentials ofthe two ends of the resistance network 38, 33, 40 are nov longer equaland a reading is obtained on the instrument 41 which is a directindication of the amount of unbalance in the space current, which inturn is a function of the movement of the armature H. The potentiometer45 provides a sensitivity adjustment and may be set to produce a fullscale movement of the instrument for a predetermined range of movementof the armature The choke 45 prevents the alternating component frompassing to the indicator circuit. The resistances 33 and 35 are of avalue to stabilize the operation of the system.

In the embodiment shown in Fig. 2 the system is adapted to the directmeasurement of unknown resistances, capacities and inductances. In thisembodiment the parts corresponding to those of Fig. 1 have been giventhe same reference characters. However, a switch 50 is provided forselectively connecting the coil H or an unknown coil into the circuit; aswitch 52 is provided for selectively connecting condenser I3 or anunknown capacity |6' into the circuit, switch 53 is provided forselectively connecting the grid leak resistance 3| or an unknownresistance 3| into the circuit and switches 54 and 55 are: provided forselectively connecting the known resistance 38 or unknown resistance 38into the circuit.

In the operation of this system the switches 50, 52, 53, 54 and 55 arefirst set to connect the known elements into the circuit and suitableadjustment is made by means of the armature l4 and the tap 4| of thepotentiometer 40 to obtain a balanced operating condition as indicatedon the indicating instrument 41. When a zero indication has beenobtained as above indicated, the appropriate switch 50, 52, 54 or 55 isactuated to connect the unknown element into the circuit,

leaving the other elements unchanged. If, forexample, the inductance ofan unknown coil II is to be measured the switch 50 will be actuated anda balance will be reestablished by varying the inductance of the coilIII or by changing the setting of the potentiometer 40. In the formercase the coil l0 constitutes a standard element, the inductance of whichfor various settings is known and may be indicated on a suitable scale,or if the balance is restored by means of the tap 4| of thepotentiometer 40 a previously calibrated scale is provided on which theinductance of the unknown element may be read.

In case the capacity of an unknown element It is to be measured, thesame procedure is adopted and the capacity of the condenser I5 is variedto restore a balance. The condenser IS in this case constitutes astandard, the capacity of which may be indicated on a suitable scale.

For measuring the value of an unknown resistance ,3 the above procedureis varied by adjusting the value of the gridleak resistor 30 until abalance is restored. In this case the gridleak resistor 30 constitutes astandard resistance, the value of which may be determined from asuitable scale.

If the value of a resistance 38' is to be determined a similar procedureis adopted varying the value of the resistance 39 which in that casebecomes a standard.

It will be noted that resistances up to the order of 50,000 ohms may bemeasured in the place indicated by the unknown resistance 38' andresistances from that point up to the value of several megohmsmay bemeasured in the location indicated by the resistance 3|. Hence, by asuitable substitution of elements the value of any high or lowresistance'may be obtained by a direct reading and without anymathematical calculations. Likewise, the value of an unknown inductanceor capacity may be obtained by a direct reading. It is to be understoodthat suitable standard resistor blocks may be incorporated for theresistors 30 and 33 and that the inductance I and condenser I mayrepresent networks of known values which may be built up to the desiredcapacity and inductance values.

The system shown in Fig. 3 comprises a head A having variable impedancecoils IOA and HA provided with an armature I4A similar to thecorresponding elements of Fig. 1 and a head B comprising coils I013 andHE and an armature I'4B of similar construction to the head A. The coilsIOA, I03 and HA, I IB are respectively connected by switches 60 and GIin circuit with condensers I5 and IE to form tuned circuits as abovedescribed. The switches 60 and BI are connected to be operated in unisonso that either the head A or the head B may be connected in circuit asdesired.

The tuned circuits are connected by lines 62 and 03 to the grids 20- and22 of oscillator tubes 24 and 25 respectively which are similar to thecorrespondingly numbered tubes of Fig. 1. The grid returns are connectedthrough fix d resistors 64 and 65 respectively and adjustable biasresistors 66 and 61 respectively and a common resistor 68 to tans 63Aand 69B.on potentiometers A and 103 respectively. The cathodes 2I' and23 of the tubes 24 and 25 are connected by switches 1| and 12 toopposite ends of either the potentiometer 10A or, the potentiometer 103.The switches 1| and 12 are preferably connected for operation in unisonand in unison with the switches 60 and 6| so that the potentiometer 10Ais connected in circuit with the head A and the potentiometer 10B isconnected in circuit with the head 13.. In this way z ro adjustments maybe made on each potentiometer to correspond with the characteristics ofthe particular head with which it is associated.

The plates 26 and 21 of the tubes 94 and 25 are connected across aresistor assembly including fixed resistances 38 and 39 and apotentiometer 40 similar to the correspondin elements of Fig. 1. Thevariable'tao M of the potentiometer 40 is connected by a line 13 to asuitable source of plate potential, the negative termi al of which isconnect d by a line 14 to the junction between the resistor 68 and theresistors 66 and 61 so that the resistor 68 constitutes a source ofcathode bias.

The feedback network comprising the condens-a ers 32 and 34 andresistances 33 and 35 is cross connected between thegrids and plates ofthe tubes 24 and 25 as in -Fig.'1.' By-pass condensers 15 and 16 areconnected across the respective grid return resistors tothe cathodes 2|and 23 respectively to eliminate the alternating current component fromsaid resistors.

In the embodiment of Fig. 3, a stage of ampliflcations is providedwhichcomprises tubes 80 6 and 3| having grids 82 and 83 connectedrespectively by lines 84- and 85 to the grid side of resistors 63 and 61and having cathodes 86 and 81 connected by a line 83 to the plate returnline 14. Tubes 80 and 8| are provided with plates 30 and 3I betweenwhich are connected resistors 92 and 93, the mid point of whichis'conziietited by line 34 to the source of plate poten- The platesideof the resistor 32 is connected through a choke I00 to a double poleswitch IOI having contacts. connected to potentiometers I'02A and I 02Brespectively, the opposite sides of which are connected by line I04 tothe plate side of the resistor 93. The potentiometers I02A and I02B areprovided with taps I05A and |05B which are connected respectively tocontacts I06A and I06B of a double switch I01, the blade 02 which isconnected by line I08 to a blade I03 of a triple pole double throwswitch IIO.

A-pair of potentiometers SA and |I5B have one side connected by a line 6to the plate side of the resistor 33 and have their other ends connectedto contacts II1A and |I1B of a double throw switch I I 8, the blade ofwhich is connected by a line I20 to a contact I2I of the switch '0. Thepotentiometers |I5A and 53 are provided with taps I 22A and I 223 whichare connected respectively to contacts I 23A and I23B of a .dou-

ble throw switch I24, the blade of which is connected by a line I25 to acontact I26 of the switch H0. The switches IOI, I01, I24 and H8 areconnected for actuation in unison and in ,unison with the "witches 60and GI so as to connect the A elements in circuit when the A head isused and the B elements when the B head is used.

The blade I30 of the switch 0 is connected b a line I3| to an indicatinginstrument I 32 the return from which is connected by a line I33 to theline I04. are connected together by a line I35.

The instrument I32 is shown as provided with a pointer I cooperatingwith a pair of scales f 4| and I42 which are adapted to be illuminatedrespectively by lights I43 and I44. The lights I43 and I 44 areconnected respectively by lines I45 and I46 to contacts I41 and I48 ofthe switch IIO, the blade I 49 of which is connected by areturn line I50through a current source. shown as a battery I5I, to the return line I52of the lights I43 and I44 so that the lights are selectively energizedin accordance with the position of the switch II 0.

It is to be understood that the switches 60, 6|, 1|. 12, MI, I01, I24and H8 are preferably arranged in the form of a gang switchwith aunicontrol so that they areoperated in unison to the A position or the Bposition. When in the A position the measuring head A and thepotentiometer 10A, I02A and II5A are connected in circuit.

Under these operating conditions, the tubes 24 and 25 operate as apush-pull oscillator in the manner explained in connection with Fig. land, with the armature MA in its mid position, the tap 63A of thepotentiometer 10A is adjusted to compensate for any mechanicalvariationin the coils IOA and II A so as to produce balanced operatingconditions at the grids 20 and 22. The tap 63A accordingly constitutes azero set which compensates for variations in the heads and in the tunedcircuits.

Contacts I34 of the switch IIO.

on the indicator instrument I32. The tap 4I thus constitutes a zero setto compensate for variations in the oscillator tubes or amplifier tubesand the associated circuits.

With the gang switch in the B position, the head B and the potentiometer10B, 1023 and II5B are connected in circuit. The tap 09B of thepotentiometer 10B is then adjusted in accordance with thecharacteristics of the B head in the same manner as the tap 69A abovementioned. Thereafter, the taps 69A and 69B remain fixed and provideautomatic compensation for variation between the heads so that nofurther adjustment is required when shifting from one head to the other.

With the grid return resistances 99 and 61 connected in the grid returnlines as shown in Fig. '3 and by-passed for alternating currents bycondensers I5 and I6 alternating potentials on the grid are eliminatedand the potential drop across the resistances due to variations in gridcurrent produce a variable bias on the grids of the respective tubes 24and 25. This voltage drop across the resistances 66 and B1 is applied tothe grids of the amplifier tubes 80' and 8| to control the spacecurrents in the respective amplifier tubes. Any unbalance in spacecurrents in the amplifier tubes produces a voltage diilerential at theopposite ends of the resistor network 92, 93 which is measured by theattenuation network including potentiometers I02A and I028, SA and H53and produces a deflection of the indicating instrument I32.

In the embodiment shown, the attenuation network is arranged for a highor a low sensitivity position and corresponding scales MI and I42 areprovided on the instrument I42. The switch H is connected to change fromthe high sensitivity network to the low sensi ivity network and at thesame time to illuminate the corresponding scale I or I42. With theswitch H0 in the L position, corresponding to low sensitivity, and withthe gang switch in the A position, the potentiometer I A is connecteddirectly across the resistance network 92, 93 and the instrument I32 isconnected across the variable tap I05A of the potentiometer I02A by acircuit including the switch I01, line I08, blade I09 of the switch IIO,contacts I34 and connecting line I35, blade I30, line I3I to theinstrument I32, thence by the return line I 33 to the lower end of thepotentiometer I02A. At the same time the blade I49 closes a circuit fromthe battery I5I to the lamp I43 which is arranged to illuminate the Lscale "I. the B position the same circuit is established to place theinstrument I32 across the variable tap I05B of the potentiometer I02B.

Under the above conditions the potentiometers I02A, I02B constitutesensitivity controls and the position of the taps I05A, I053 determinesthe portion of the potential drop across the potentiometer which isapplied to the indicating instrument I32. These taps will normally beadiusted so that a predetermined movement of the armatures I4A or I4Bwill produce a full scale movement of the pointer I40 of the instrumentI32.

With the switch H0 in the H position, the instrument I 32 instead ofbeing connected across the tap of the potentiometer I02A or I 023, isconnected across the tap II5A or H53, and the potentiometers II5A andII5B are connected respectively across the variable taps of thepotentiometers I02A and I02B. With the gang switch in the A position,the circuit may then be traced With the gang switch in i thereby placingthe potentiometer II5A directly across the, variable tap I05A of thepotentiometer I02A. The circuit to the measuring instrument I32 may betraced from the tap I22A of the potentiometer II5A, contact I23A, switchI24, line I25, contact I26, blade I30, line I3I to instrument I32;thence by the return line I33, line I04 and line H6 to the other side ofthe potentiometer II5A, thereby placing the instrument I32 across thevariable tap I22A of the potentiometer IIIA. At the same time, the bladeI49 completes a circuit from the battery I5I to the light 4' to illu-*minate the H scale I42 of the instrument I32. In

this position the taps I05A and I22A constitute the sensitivity controlandare adjusted to produce a full scale movement of the pointer I40 inresponse to a predetermined movement of the armature I4A which may besubstantially less than the movement measured with the switch I I0 inthe low sensitivity position. Adjustment of the grid leak resistances,and 61 also provides a sensitivity control, the sensitivity being higherwhen the resistance is higher and vice versa. With the gang switch inthe B position the same circuits are established to connect thepotentiometer II5B to t e tap M513 and the instrument I32 to the tapI22B.

The lights I 43 and '4 may be placed in separate compartments toselectively illuminate the respective scales I4Iand I42, or if desiredthe scales may be made of complementary colors such as red and green andthe lights I43 and I44 may be of corresponding colors so that only onescale is rendered visible at a time.

It has been found that the above circuit provides extreme sensitivityand may be used to measure movements of the armature I 4A- or B of theorder of millionths of an inch.

The use of an attenuation network, including the second potentiometers,permits the circuit to be adjusted for high sensitivity without drawingexcessive current from the plate circuits of the tubes and 8| whichwould tend to make the response non-linear. the A. C. components fromthe measuring circuit.

When the impedances of the two coils IIIA and I IA or I03 and I IB areequal, equal voltages are applied to the grids 20 and 22 and equal spacecurrent is drawn by the two tubes. However, when the impedance of one ofthe coils decreases and that of the other coil increases, the grid biasof the tubes change correspondingly because one of the tubes will thendraw less grid current than I the other due to the smaller A. C. swingand that tube'will pass a correspondingly less space current. The mutualcoupling between the two co ls I0 and II through the common armature I4Atends to lock the oscillators in step, although. the coupling may bequite loose. However, the cross co nect on between the plates and gridsof the two tubes tends to keep the oscillators in step Without mutualcoupling. The mutual coupling in the grid circuit, however, is moreefiective for holding the oscillators in step than an equal amount ofcoupling in the plate circuit. Consequently, the coupling produced bythe armature I4 is of importance in stabilizing the operation.

In the system shown, two sets of attenuation networks are provided sothat they may be individually adjusted in accordance with the particularhead to be used for measuring purposes. This The choke I 00 eliminatesMamas which are to be checked, in which event the heads will be arrangedto be engaged by the respective 1 vidual tuned input circuits, eachincluding an inparts either simultaneously or in sequence,- and afterplacing the parts in position it is only necessary for the operator toactuate the gang switch for connecting first the A head and then the Bhead in circuit, in each instance noting the measurement on thecorresponding scale of the instrument I32. For routine checking purposesthe scales are preferably arranged as a limit scale with a mark toindicate the permissible limit of movement of the pointer I40, anymovement of the pointer beyond these points indicating an excessivevariation in the dimensions of the parts.

The instrument may be adjusted so that ordinarily a measurement may betaken with the high sensitivity scale. If, 'however, the variation indimensions of the part is such that when measured on this scale thepointer I40 moves entirely across the corresponding scale of theinstrument I32 the switch llll may be changed to the low sensitivityposition and the reading of the dimension of the part may be taken onthe low sensitivity scale. In this way the various readings may beaccurately and rapidly taken with a minimum of mechanical manipulationof the parts.

It is to be understood, of course, that any desired number of headsmaybe provided and that a corresponding set of attenuation networks willbe connected to be placed in circuit by the gang switch. The gang switchmay be automatically operated to connect the various heads in circuit inrecurring sequence. The system accordingly provides for the taking of aplurality of readings by means of a Single indicating instrument withoutduplicating the main parts of the measuring circuit. At the same timethe circuits are individually adjusted in accordance with the parts tobe measured with the individual measuring heads.

The above system may be used as indicated in Fig. 2 for measuring anyelement of the circuit or for checking tubes. The system has theadvantage of a low power input to the grid which permits the use of asmaller head and reduces the power required for actuating the styluswhich controls the armature l4. At the same time the system has a highdegree of stability due to the balanced effects of the tubes. .Theindividual tube circuits for the two oscillator tubes with the crosscoupling make it possible to vary individu ally the LC ratio of each ofthe input circuits so as to obtain a minimum harmonic output when thetubes are in balance.

Although certain specific embodiments of the invention have been shownfor purposes of illustration, its to be understood that the invention iscapable of various uses as will be apparent to a person skilled in theart. The invention is only to be limited in accordance with the scope ofthe following claims.

What is claimed is:

1. An electric measuring system comprising a push-pull oscillatorincluding a pair of tubes having cross-connected feedback circuits forcausin said tubes to oscillate in unison and having indiductance and acapacity, said inductances being variable in opposite sense in responseto a quantity to be measured to produce a corresponding change in thespace currents drawn by the two tubes, and means responsive to theunbalance in said space current.

2. An electric measuring system comprising a push-pull oscillatorincluding a pair of tubes having cross-connected feedback circuits forcausing said tubes to oscillate in unison and having individual tunedinput circuits, each including an inductance and a capacity, saidinductances having mutual coupling and being variable in opposite sensein response to a quantity to be measured to produce a correpondingchange in the space currentdrawn by the two tubes, and means responsiveto the unbalance in said space currents.

3. An electric measuring system comprising a push-pull oscillatorincluding a pair of tubes having cross-connected feedback circuits forcausing said tubes to oscillate in unison and having individual tunedinput circuits, each including an inductance and a capacity, saidinductances being variable in opposite sense in response to a quantityto be measured to produce a corresponding change in the space currentsdrawn by the two tubes, and an indicator connected to measure theunbalance in said space currents.

4. An electric measuring system comprising a push-pull oscillatorincluding a, pair of tubes having cross-connected feedback networks forcausing said tubes to oscillate in unison and having individual tunedinput circuits, each including an inductance and a capacity, saidinductances being variable in opposite sense in response to a quantityto be measured to produce a corresponding change in the space currentsdrawn by the two tubes, a resistance network interconnecting the outputcircuits of said tubes to produce a voltage differential in response todifferential space currents and a measuring circuitconnected to measuretthe voltage difierential across said network.

5. An electric measuring system comprising a push-pull oscillatorincluding a pair of tubes having intertu'be coupling means to cause saidtubes to oscillate in unison and having individual tuned input circuitseach including a variable impedance device, said last devices having acommon armature variable in response to a quantity being measured tovary the impedances of said devices in opposite sense, and producecorresponding changes in the space currents drawn by said tubes, and ameasuring circuit connected to measure the unbalance in space currents.

6. An electric measuring system comprising a push-pull oscillatorincluding a pair of tubes having intertube coupling means to cause saidtubes to oscillate in unison and having individual tuned input circuitseach including a variable impedance device, said last devices having acommon armature variable in response to a quantity being measured tovary the impedances of said devices in opposite sense and producecorresponding changes in the space currents drawn by said tubes, ameasuring circuit connected to measure the unbalance in space currentsand adjusting means to equalize the space currents in said tubes whensaid armature is in a mid-position.

'7. In an electric measuring system a push-pull oscillator comprising apair of tubes having grids and plates, 9, feedback coupling including acondenser and resistor connected in series between the grid of each tubeand thetplate 0f the other UNITED STATES PATENTS ubes, individual tunedinpu circuits for said tubes including inductances and capacities conggf T Name nected in parallel, said inductances being variable 2'111235 fMay 9 8 in opposite sense in response to the quantity be- 5 v 1 3 ingmeasured self biasing resistors in th 2'149756 Arenberg et 1939 I 6 grid2 208 329 M e] k Jul 16 1940 return circuit of each tube connected tocause 2338732 y 1944 the space currents drawn by said tubes to vary2367'965 3; g 45 with said changes in inductances, and means 2:371395 gz513 connected to respond to an unbalance between 10 2,415,773 vnkomersonFeb. 11, 1947 said space currents.

CHARLES F. CQAKE.

REFERENCES CITED The following references are of record in the 15 fileof this patent:

